The invention relates to a method for producing powder particles that are uniformly coated with functional groups and in particular glass powder, glass ceramic powder and/or ceramic powder.
Such powders find wide application in a large sector of technology and are obtained by coating a crude powder. For example, such powders in the form of glass powders are used in, among other fields, dental technology to prepare filled synthetic resins and in cosmetic and pharmaceutical formulations. They are also used both externally and internally in paints, lacquers, plasters and wood preservatives and as fillers in adhesives to increase their hardness or reduce their stretching properties. The preparation of, in particular, fine-particle powders with small or very small particle size is described, for example, in U.S. Pat. No. 6,010,085. Moreover, the preparation of very fine contaminant-free (high-purity) glass powders having a mean particle size d50 of 0.2 μm to 10 μm is known from U.S. Pat. No. 5,340,776, the d50 value of 0.2 μm meaning that 50% of all particles are smaller than 0.2 μm. By this method, a glass powder with a maximum particle size of ≦0.300 μm is comminuted in an agitated ball mill or attritor mill with grinding elements consisting of glass the spallings of which do not affect the properties of the glass powder obtained, for example its refractive index. By this grinding method, the grinding is done in the presence of a grinding liquid until the desired particle size is attained. The grinding slurry is then frozen, and the grinding liquid is removed from the grinding slurry by freeze-drying.
If such powders are used as fillers for plastic materials employed in dental technology, it is important that the refractive index of the glass powder very closely match that of the plastic material so as to render the filled plastic material highly transparent and translucent. If the glass powder contains particles of contaminants such as spallings from the grinding mill that have a different refractive index, however, the translucency and transparency and possibly the color of the filled plastic material deteriorate so that often the powder can no longer be used or its use is severely restricted.
For this reason, as described in U.S. Pat. No. 6,010,085 and U.S. Pat. No. 5,340,776, all parts exposed to abrasion, such as the mill container and the grinding chamber and agitator are coated with the same material that constitutes the powder or with a plastic coating that can readily be removed by extraction, distillation or pyrolysis. When the glass particles are used as filler for plastic materials, the surface of the glass powder particles is usually/often coated with an appropriate silane to achieve better incorporation of the powder into the plastic matrix. Such a method is described, for example, in PCT/US92/04553. The silanization is accomplished by first dissolving the silane in an appropriate solvent and prehydrolyzing it and then applying it, for example in a mixer such as a planetary mixer or in a vibration or drum mill, onto the previously fully ground glass powder. During this procedure, however, the silanes also form dimers, trimers or even oligomers which not only form silanol agglomerates but because of the remaining silanol groups on the partly coated particles form aggregates with a size of up to 100-300 μm. With the aforesaid mixers or mills, the formation of these agglomerates or aggregates cannot be prevented. Even subsequent grinding of the agglomerates does not yield usable products.